This invention relates to a digital watermark embedding method and its related art that are capable of improving the evidential reliability of digital image data.
With the spread of digital still cameras and video cameras, in addition to conventional and widely used silver bromide photography, digital image data (hereinafter designated simply “image data”) is now easy to use.
Image data can be input into an information-processing device, such as a personal computer, can be edited using image processing software. For example, part of the image data can be cut off and replaced with other images. With digital image processing, a level is reached in which even the eyes of a professional are unable to discover whether an image is an edited image or an entirely original image.
On the other hand, it is very difficult to edit an image taken with silver bromide photography. In other words, the probability is very low that objects on a silver bromide photograph are falsified. Accordingly, it may be said that the silver bromide photography has high credibility and high evidential reliability.
In contrast, it must be said that digital image data can be easily falsified by editing. Accordingly, the evidential reliability of a digital image is low in the absence of measures to improve the credibility of the digital image data.
Cases where the characteristics of the image data are abused never cease. For example, let it be supposed that an offender X has maliciously falsified image data for the purpose of casting aspersions on a person A. First, the offender X prepares an undesirable base image (for example, an image of violent or obscene scenes) containing an image of a person A. Then the offender X substitutes only a face part of another person on this base image for the face of the person A. Thereafter, the offender X posts this image on a web site of the Internet to slander the person A in support of a false story about the person A. Besides such defamatory activity, other crimes or unfair acts of falsifying image data are now made possible by the ease with which a digital image can be edited.
In response to this problem, there are demands for a technique capable of judging whether image data has been falsified or not, and capable of preventing the falsification beforehand, and, additionally, capable of improving the evidential reliability of digital image data.
As a solution thereto, a technique in which a digital signature is attached to image data can be mentioned (see Japanese Unexamined Patent Publication Nos. Hei-11-215452 and Hei-11-308564), the disclosure of which is hereby incorporated by reference.
However, in light of these references, the digital signature can be easily removed from the image data. With the digital signature removed, a judgment on falsification becomes impossible. Further, the amount of image data is increased by the data of the digital signature. As a result, when the amount of digital data storage is limited, the number of sheets of images that can be recorded is reduced.
Further, in most cases, image data is irreversibly compressed and encoded, before recording. Because the image data before compression is different from the image data that has been compressed, if a digital signature is applied to image data before compression/encoding, the digital signatures on the original and the compressed image data in the two cases are different whether or not the digital data has been falsified. For this reason, if the image data is compressed, the digital signature is applied to image data after compression/encoding.
If the image data is falsified before compression, a decision on whether or not falsification has occurred cannot be based on the digital signature. Further, each time compression/extension processing is repeated, a new digital signature must be added to the compressed digital data. This is not practical.
Thus, the technique of attaching a digital signature to image data doesn't give sufficient evidential reliability to the image data.
In consideration of the foregoing, the idea of embedding a digital watermark into image data itself and making a judgment on falsification on the basis of this digital watermark can be proposed as a technique in which an error rate is reduced to a negligible extent in spite of the fact that compression/extension is performed or errors occur in data transmission. Additionally, the following two respects are needed practically.
(A) The embedding mechanism of the digital watermark is not easily understood. If the embedding mechanism is easily understood, there is a chance that the digital watermark itself might be falsified, with a resulting reduction in its credibility.
(B) The amount of data to be embedded should be as small as possible. This is for economical efficiency in information processing.
Referring to FIG. 6, as disclosed in Japanese Unexamined Patent Publication No. Hei-11-85550, the point hereof will be hereinafter described in detail. It is assumed that an embedding data length is 32 bytes (256 bits). A code term length is 31 bits containing 21 information bits and 10 redundancy bits for error correction.
First, when recording with an embedded digital watermark, the following steps are executed.
(1) The embedding data (256 bits) is subdivided for each information bit (21 bits). Herein, the end of the embedding data situated at the 13th code term has only four bits. Four bits is smaller than the number of information bit (21 bits). Padding (dummy data) of either “1” or “0” is applied to all the remaining bits (21−4=17 bits).
(2) Thereafter, redundancy bits (10 bits) of the 1st to 13th code terms are added. Then 1st to 13th code terms after the redundancy bits are added are brought together into one as unit data.
(3) Thereafter, information obtained by repeating the unit data three times for a later majority decision (three sets in the total of the 1st to 13th code terms, 14th to 26th code terms, and 27th to 39th code terms) is defined as real embedding information.
(4) The real embedding information is embedded into image data, and it is stored on a recording medium.
Next, when reproduced (i.e., when the digital watermark is extracted), the following steps are executed.
(a) The image data and the real embedding information are separated and extracted from the recording medium.
(b) Based on the image information, reproduction is carried out.
(c) The real embedding information is divided into three parts, and the same unit data, repeated three times, are extracted.
(d) The error correction of a corresponding information bit is carried out by a redundancy bit.
(e) A majority decision is made regarding the corresponding bit of each unit data that has been extracted, and error correction is performed. For example, a majority decision regarding the first bit of the information bit is made by each first bit of the 1st, 14th, and 27th code terms.
(f) The result of the majority decision is used as embedding data.
Indeed, according to this procedure, the error rate is reduced to a negligible extent in spite of the fact that compression/extension is performed or errors occur in data transmission.
However, the two respects of (A) and (B) mentioned above are not satisfied. That is, since the same unit data is simply repeated three times, high regularity is exhibited, and therefore the embedding mechanism is easily understood. For this reason, there is a fear that the embedded digital watermark itself will be falsified, and the recorded digital data still lacks sufficient evidential reliability.
Further, since dummy data that has been subjected to padding many times appears, there is much useless labor, and this is disadvantageous from the viewpoint of economical efficiency in information processing.